spi/spi-xilinx: Fix mixed poll/irq mode

Once the module process a transfer in irq mode, the next poll transfer
will not work because the transmitter is left in inhibited state.

Fixes: 22417352 (Use polling mode on small transfers)
Reported-by: Edward Kigwana <ekigwana@scires.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org